The present invention relates to electrochemical fuel cells. In particular, the invention provides an improved membrane electrode assembly for a fuel cell, and a method of making an improved membrane electrode assembly. The improved membrane electrode assembly comprises integral fluid impermeable seals and coextensive electrode and membrane layers.
Electrochemical fuel cells convert reactants, namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Solid polymer fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers comprising porous, electrically conductive sheet material. The membrane is ion conductive (typically proton conductive), and also acts as a barrier for isolating the reactant streams from each other. Another function of the membrane is to act as an electrical insulator between the two electrode layers. The electrodes must be electrically insulated from each other to prevent short-circuiting. If a multi-layer MEA is cut, tiny portions of the electrically conductive electrode material, such as stray fibers, may bridge across the thin membrane, interconnecting the electrodes, which could cause electrical short-circuiting in an operating fuel cell. Conventional MEAs incorporate a membrane with a larger surface area than the electrode layers, with at least a small portion of the membrane extending laterally beyond the edge of the electrode layers. The protruding membrane edge helps to prevent short-circuiting between the electrodes around the edge of the membrane. A problem with this is that it is difficult to cut an MEA after the electrodes have been joined to the membrane so that the thin membrane has a larger area than the electrodes. A conventional MEA is fabricated by manufacturing and cutting the electrodes and membrane layers separately. After the electrodes and membrane have been cut to the desired size and shape, the cut electrode layers are laminated with the cut membrane layer. These steps are not conducive to high speed manufacturing processes. It would be preferable to manufacture a sheet or roll of MEA material that already comprises the electrode and membrane layers, wherein this multi-layer material could then be cut to the desired size and shape for individual MEAs. An MEA cut in this way, such that the electrodes and membrane are coextensive, is described herein as being a xe2x80x9cflush cutxe2x80x9d MEA. However, this approach has heretofore been impractical because of the short circuiting problem described above.
In a fuel cell stack, the MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluid streams. The plates act as current collectors and provide support for the electrodes. To control the distribution of the reactant fluid streams to the electrochemically active area, the surfaces of the plates that face the MEA may have open-faced channels or grooves formed therein. Such channels or grooves define a flow field area that generally corresponds to the adjacent electrochemically active area. Such separator plates, which have reactant channels formed therein are commonly known as flow field plates. In a fuel cell stack a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, one side of a given plate may serve as an anode plate for one cell and the other side of the plate may serve as the cathode plate for the adjacent cell. In this arrangement the plates may be referred to as bipolar plates.
The fuel fluid stream that is supplied to the anode typically comprises hydrogen. For example, the fuel fluid stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol may be used. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air. In a fuel cell stack, the reactant streams are typically supplied and exhausted by respective supply and exhaust manifolds. Manifold ports are provided to fluidly connect the manifolds to the flow field area and electrodes. Manifolds and corresponding ports may also be provided for circulating a coolant fluid through interior passages within the stack to absorb heat generated by the exothermic fuel cell reactions.
It is desirable to seal reactant fluid stream passages to prevent leaks or inter-mixing of the fuel and oxidant fluid streams. Fuel cell stacks typically employ resilient seals between stack components. Such seals isolate the manifolds and the electrochemically active area of the fuel cell MEAs by circumscribing these areas. For example, a fluid tight seal may be achieved in a conventional fuel cell stack by using elastomeric gasket seals interposed between the flow field plates and the membrane, with sealing effected by applying a compressive force to the resilient gasket. Accordingly, it is important for conventional fuel cell stacks to be equipped with seals and a suitable compression assembly for applying a compressive force to the seals.
Conventional methods of sealing around plate manifold openings and MEAs within fuel cells include framing the MEA with a resilient fluid impermeable gasket, placing preformed gaskets in channels in the electrode layers and/or separator plates, or molding seals within grooves in the electrode layer or separator plate, circumscribing the electrochemically active area and any fluid manifold openings. Examples of conventional methods are disclosed in U.S. Pat. Nos. 5,176,966 and 5,284,718. Typically the gasket seals are cut from a sheet of gasket material. For a gasket seal that seals around the electrochemically active area of the MEA, the central portion of the sheet is cut away. This procedure results in a large amount of the gasket material being wasted. Because the electrodes are porous, for the gasket seals to operate effectively, the gasket seals ordinarily are in direct contact with the flow field plates and the ion exchange membrane. Therefore, in a conventional MEA, electrode material is cut away in the sealing regions so that the gasket will contact the ion exchange membrane. Some MEAs use additional thin-film layers to protect the ion exchange membrane where it would otherwise be exposed in the gasket seal areas. Separate components such as gasket seals and thin-film layers require respective processing or assembly steps, which add to the complexity and expense of manufacturing fuel cell stacks.
Accordingly, it is desirable to simplify and reduce the number of individual or separate components involved in sealing in a fuel cell stack since this reduces assembly time and the cost of manufacturing.
An improved MEA for an electrochemical fuel cell comprises:
(a) a first porous electrode layer;
(b) a second porous electrode layer;
(c) an ion exchange membrane interposed between the first and second porous electrode layers wherein said first and second electrode layers and said membrane are coextensive;
(d) a quantity of electrocatalyst disposed at the interface between the ion exchange membrane and each of the first and second porous electrode layers, thereby defining an electrochemically active area on each of the first and second electrode layers; and
(e) a resilient fluid impermeable seal integral with the MEA, the seal comprising a fluid impermeable sealant material impregnated into the first and second porous electrode layers in sealing regions thereof and extending laterally beyond the membrane and the electrode layers to an external region,
wherein the sealing regions comprise regions that circumscribe the electrochemically active area, and wherein the external region comprises regions that circumscribe external manifold openings therein.
In a preferred embodiment the sealing regions comprise regions that circumscribe the electrochemically active area of the electrode layers. Preferably the sealant material impregnates a portion of the MEA electrodes in the peripheral region and extends laterally beyond the edges of the electrode layers and membrane (that is, the sealant material envelops the membrane edge).
Where the MEA further comprises one or more openings formed therein, such as opening for a tension member, the sealing region further may comprise regions that circumscribe such openings.
In a fuel cell stack the sealing regions cooperate with the fuel cell separator plates to prevent fluids from leaking around the edges of the MEA. The sealant material is preferably an elastomer. In a preferred method of making an improved MEA, the sealant material is injection molded. Accordingly, it is desirable for the uncured sealant material to be flow processable. After the uncured sealant material has been applied to the MEA, it is allowed to cure to form a resilient elastomeric material. The elastomeric sealant material may be a thermosetting material, as long as the curing temperature is compatible with the MEA components, and in particular, the ion exchange membrane.
The sealant material may also be used to form a reference feature such as a raised edge or protrusion for assisting in the assembly of the fuel cell. For example, when an outer perimeter edge seal is being molded, at least one of the edges could be molded with a reference edge, which can be used to align the MEA during manufacturing processes. Alternatively, a protrusion such as a cylindrical plug could be molded in a location, which can be aligned with a corresponding cylindrical depression in an adjacent separator plate during stack assembly.
In another embodiment of the present membrane electrode assembly, the sealing regions comprise regions that circumscribe and extend into the electrochemically active area, and at least a portion of the sealing regions overlay reactant distribution ports of an adjacent fluid flow field plate. The sealing regions may assist in directing the flow of reactants to the flow field channels of the flow field plate and may protect the portion of the MEA overlaying reactant ports in the plate.